I am thinking about installing one 10" sub behind the back seat and making it almost level to the floor. How will the sound quality be? Do you think that putting it there will hurt the quality?

I prefer it there that way I won't sacrifice the extra space in the scion and I can still throw a lot of stuff in my car.

I was also thinking of putting plexiglass. Anybody have experience with that?

It will look sort of like this:

 

A lot of people have done that.... as long as the enclosure under the cover is sealed well (or tuned if ported) and the specs of the enclosure meet the requirements of the sub, it'll sound just as good as a standard box...

 

where can you get that!

 

losing the spare would suck hard. and where would you put the amp or amps?

 

Are you looking for an xB? That pic is of an xA, your registry shows an xB.

If you are looking for an xB, check out audioformz.com. They have a fiberglass enclosure which fits right in the storage compartment on the right. Spare tire stays intact. I have one and love it.

I built a box to fit in the same space, but it's very tough to get the necessary volume, and there is a bump in the floor which really sucks for fitting a box back there.

 

I'm a futre xA owner and am wondering about sub placement that will leave room for cargo.

 

I am in the process of building my own fiberglass box (similar to the audioformz box) that will house a 12" rf punch sub. I used the foam storage box as the template to form the fiberglass. I'm into the project about 30.00$ not including sub ao amp. . This box will still allow me to use the storage cover and leave the floor flat for anything that i want to put into my cargo area without puting a hole in my speaker. I will post pics as soon as it's finished.